1. Field of the Invention
The present invention relates to an image-forming apparatus that forms an image using a developing-material and to a control method thereof.
2. Description of the Related Art
Generally, an electrophotographic image-forming apparatus forms an image by transferring a developing-material image (toner image) formed on the surface of a photosensitive drum, serving as an image carrier to a transfer material serving as a transfer medium.
There are also various systems for supplying developing-material. For example, with a process cartridge system, the photosensitive drum and the developing-material container are integrated, and the process cartridge needs to be replaced when the developing-material runs out. On the other hand, a developing-material replenishing system is known in which the developing container is newly replenished with developing-material when the developing-material runs out. With the developing-material replenishing system, a removable developing-material replenishing container is provided in the image-forming apparatus, and the developing container is replenished with developing-material from the developing-material replenishing container. More specifically, inventions have been proposed that have a developing-material circulation route in order to supply developing-material to a developing member (developing roller) after the developing-material inside the developing container and the developing-material newly replenished from the developing-material replenishing container have been sufficiently mixed inside the developing container. Japanese Patent Laid-Open No. 8-30084 (Patent Document 1) and Japanese Patent Laid-Open No. 2006-99043 (Patent Document 2) are examples thereof.
However, the following problems exist with the prior art. Generally, inside a developing apparatus, developing-material is repeatedly supplied to and eliminated from the developing roller. The charging characteristics deteriorate quite considerably during the repeated circulation of the developing-material inside the developing apparatus. This is because of the resin constituting the developing-material being abraded or deformed by mechanical rubbing. Deterioration of charging characteristics may also be caused by external additives that have been added to the surface of the developing-material separating or becoming embedded within the resin.
On the other hand, replenishing the developing container with new developing-material from the replenishing container means that newly replenished developing-material gets mixed with deteriorated developing-material inside the developing apparatus. The developing-material is carried inside the developing apparatus from one end to the other, and supplied to the developing roller. Consequently, developing-material with different charging characteristics ends up adhering to the developing roller on the upstream side and the downstream side in the carrying direction of the developing-material. This is not desirable since it causes density nonuniformity in the developing-material image formed with respect to the electrostatic latent image on the image carrier to occur in the longitudinal direction of the developing roller (parallel to the carrying direction of the developing-material).
Further, various factors contributing to density nonuniformity in an image-forming apparatus are envisioned apart from density nonuniformity in the developing apparatus, such as an OFS optical system, for example. Accordingly, it is desired to collectively address density nonuniformity caused by the developing apparatus and other factors.